In addition to panel, backlight is the most important component determining the picture quality of a display device in the display device, the illumination effect thereof directly determines whether the quality of the display picture of the display device is good or not. With the increasing requirements of enjoyment, people's requirements on the picture of the display device are getting higher and higher. In order to meet the needs of customers, manufacturers are continuously improving the effects of the backlight, and have developed various light sources such as EL (ElectroLuminescence), CCFL (Cold Cathode Fluorescent Lamp), HCFL (Hot Cathode Fluorescent Lamp) and today's LED (Light Emitting Diode). Among these light sources, LED lamp has undoubtably become the best light source in the current display devices for its good lighting effects and low price. As shown in FIGS. 1 and 2, FIG. 1 is the front structural schematic view of an existing direct type backlight and FIG. 2 is a cross-sectional structural schematic view of an existing direct type backlight, the existing direct type backlight includes: a backplate 10, a printed circuit board 30 disposed on the backplate 10, LED lamps 33 disposed on the printed circuit board 30 by LED lamp holders 32, side frames 20 provided on the inner wall of the backplate 10, reflection sheets 40 covering the wall of the cavity formed by the printed circuit board 30 and the side frames 20, a diffusion plate 90 disposed on the side frames 20, an optical film 70 disposed on the diffusion plate 90 and outer frames 80 stuck on the outsides of the backplate 10 and the optical film 70. During operation of the existing direct type backlight, light rays emitted from the LED lamps 33 irradiate on the diffusion plate 90 and the optical film 70 through the air layer between the LED lamps 33 and the diffusion plate 90, and the point light emitted from the LED lamps 33 are diffused into surface light through the diffusion plate 90.
Defect existing in the prior art is that, in order to provide uniform brightness on the displays screen, the light rays emitted from the LED lamps 33 to the display screen must be mixed, and therefore, certain mixing distance has to be presented between the LED lamps 33 and the optical film 70. In existing direct type backlight, the LED lamps 33 irradiate the optical film 70 through the air layer and the diffusion plate 90, which causes that the thickness of the direct type backlight cannot be further reduced, and therefore the thickness of the display device is affected. In addition, as the optical film 70 is directly disposed above the air layer, the optical film 70 is apt to deform for lacking support. Light rays transmitted through the deformed optical film 70 become nonuniform, which lowers the display quality of a display device adopting the existing backlight.